The present invention relates to a system for automatically maintaining gas turbine nitrogen oxide (NO.sub.x) emissions at a specific level in parts per million by volume during all ambient conditions for no load to full load fuel flows. More particularly, the invention relates to a device and method for automatically adjusting the input water flow rate to the gas generator to maintain constant nitrogen oxide emissions substantially equal to or less than maximum permissible amounts.
Gas turbines are well-known in the art. They include a compressor which compresses the inlet air and transfers it to the combustor. The combustor has fuel added which when ignited, heats the compressed air and produces output gases which are then expanded across a turbine to provide the power output. The combustor burns fossil fuels such as natural gas, either in the gaseous or liquid state. Water is also added in the combustor to bring the combustor temperature down.
Because of the use of the fossil fuels as the combustor fuel, nitrogen oxide emissions (NO.sub.x) are found in the exhaust gases. The gas turbines are designed to maintain a government imposed minimum level of NO.sub.x by utilizing water injection in the combustor which is added at the same time as the fuel. Water cools the combustor and reduces the temperature of the output gases which reduce NO.sub.x emissions. It is difficult to maintain constant NO.sub.x emissions because of variable operating conditions of the gas turbine such as inlet air temperature, turbine RPM and various loads on the turbine, all of which cause the system to operate so as to change the NO.sub.x emissions.
Some prior art systems attempt to use manually controlled water metering valves so that they can be manually adjusted to attempt to control the amount of water being used to obtain some optimum water/fuel ratio. However, as can be imagined, it is almost impossible to manually control the water/fuel ratio to provide the desired NO.sub.x emissions with continuously changing conditions such as inlet air temperature, turbine speed, and load applied to the turbine.
Other prior art gas turbines utilize constant flow water injectors to reduce combustor temperatures. These systems are designed for full horsepower operating level and provide proper NO.sub.x emission levels only at the full horsepower operating point. Operation under conditions other than the design conditions simply wastes water and decreases the operating efficiency of the turbine.
The present invention relates to a system for estimating the desired water/fuel ratio to provide a substantially constant NO.sub.x emission as a function of corrected turbine speed, inlet air temperature, load and the amount of fuel being consumed as determined by the fuel metering valve position. Tests have shown that there is no correlation between the water/fuel ratio and the gas generator compressor inlet temperature only or only the temperature of the combustion gases between the compressor and the power turbine. However, there is a linear correlation between the water/fuel ratio and the gas generator speed corrected for inlet air temperature to maintain NO.sub.x emissions at a minimum constant level. Thus it is well known that if the inlet air temperature T.sub.2 is corrected by factor ##EQU1## and the square root of that correction factor or (.sqroot..theta.) is divided into the actual gas generator speed, AGG, a corrected gas generator speed, CGG, is obtained. Thus, ##EQU2## From this corrected gas generator speed, the desired water/fuel ratio is obtained as follows: ##EQU3## where WFR=the desired water/fuel ratio for substantially constant NO.sub.x emission, CGG=corrected gas generator speed, N.sub.l =minimum corrected gas generator speed for WFR=0.0, N.sub.2 =(corrected gas generator speed for a water/fuel ratio=1.0)-N.sub.1 and K=a constant. By multiplying the desired water/fuel ratio, ##EQU4## obtained by this calculation times the fuel flow in pounds per hour, W.sub.F, as determined by the fueI f1ow valve position, a signal may be obtained for adjusting the desired water flow valve position, W.sub.p, to obtain the proper water flow, W.sub.w. Thus, ##EQU5## Therefore, it is an object of the present invention to provide a gas turbine with substantially constant NO.sub.x emissions over variable inlet air temperatures, gas generator speeds and gas generator loads.
It is another object of the present invention to provide a gas generator with substantially constant NO.sub.x output emissions by using a variable water/fuel ratio by controlling the amount of water used depending upon the amount of fuel being used under variable inlet temperature and load conditions.
It is also an object of the present invention to determine the desired water/fuel ratio, determine the amount of fuel being utilized, multiply the desired water/fuel ratio times the amount of fuel being used to obtain the amount of water that should be used and then setting the water metering valve according to the amount of water desired to be used.